By reading the introduction of the interfaces, you obtained a mechanism that makes a class really and completely abstract.
From the article we highlighted their total abstraction.
We noticed that it's contained methods must be overridden by the inherited class(es).
Furthermore a interface does not have any attributes.
Finally we wrote - just on the rush - that the Interfaces mechanism overpasses one compiler's weak point, about the inability to support the multiple inheritance.
In brief: the multiple inheritance in FreePascal grants that an inheriting class can inherits from more than one more abstract class.
It so expands its possibility to enrich itself, always by keeping the order and the hierarchy achieved with the interfaces.
And that's very good... in theory.